This invention relates in general to sewing devices and in particular to a new and useful device for facilitating the formation of a doubly turned hem on lengths of material.
In the hem folding device, known from U.S. Pat. No. 2,004,080, the material to be sewn is led past the stitch forming site of a sewing machine by several conveyor belts, arranged next to one another, and by an external hem folding belt. A hem folding sleeve with a U-shaped inlet region and a doubly twisted outlet region for forming a doubly turned hem are arranged along the external hem folding belt. The formation of the hem is supported by the driven hem folding belt on the whole length of the hem folding sleeve in the sense that, for the formation of the first 180.degree. turn, the hem folding belt runs under the edge to be turned in the hem folding sleeve and, for the formation of the second 180.degree. turn, it follows the shape producing hem formation and finally embraces the finished hem in a U-shaped fashion in the region of the outlet.
By these means, the transfer of tensile stress to the workpiece while the machine is working is said to be avoided and the material is guided into and through the folding sleeve. The hem formation is, however, supported directly by the hem folding belt only for the fabrication of the second 180.degree. turn, while the first 180.degree. turn is formed exclusively by loading the fabric past the appropriately bent guiding surfaces of the folding sleeve, so that, in this case, there is an initial compression, which has a decisive effect on the magnitude of the hem distortion. Especially for materials with a rough or adhering surface, this distortion at the start of the hem cannot be avoided. This effect is particularly noticeable with workpieces, the inlet edge of which is already formed into a doubly turned hem. In the region of the hemmed inlet edge, nine layers of materials lie over one another at the side edge of these workpieces after the hem is formed. These form a bulge. Considerable frictional resistance must be overcome to pass this bulge through the outlet of the hem folding sleeve, because the region of the outlet of the hem folding sleeve, which gives the hem its final state, must be matched not to the bulge, but to the thickness of the material.
When using the hem folding belt of the device of U.S. Pat. No. 2,004,080, the thickness of the hem folding belt must be taken into consideration in addition to the thickness of the material. This, however, is then responsible for the fact that the side edge of the workpiece is not turned accurately and guided reliably over the whole of its length. In any case, the bulges are distorted or contorted as they are pushed through the outlet of the hem folding sleeve. The folded parts of the hem do not then lie flush with the inlet edge of the workpiece and considerably impairs the appearance of the workpiece.